Flight, May 1925

THE SIKORSKY S.29A COMMERCIAL BIPLANE

   FOR some time past M. Igor Sikorsky, the Russian aircraft designer and pioneer of the giant multi-engined aeroplane the huge Sikorsky biplanes of 1914 were, perhaps, the first successful aeroplanes employing more than two engines and having total wing spans in the neighbourhood of 100 ft. or so has been engaged in the design and construction of a medium-sized twin-engined "transport" aeroplane. This work, which has been carried out at New York, U.S.A., where M. Sikorsky and several other Russians (driven from Russia by the Revolution) have established themselves, has just recently advanced to the stage where the first flying tests have been carried out, at Roosevelt Field, L.I., by the senior aeronautical students of New York University, under the direction of Alexander Klemin, Associate Professor of the University.
   Owing to severe weather conditions the full programme of the tests scheduled could not be carried out, but sufficient data was secured to give an idea of the performance of the machine. From the accompanying illustrations it will be observed that the Sikorsky S.29A is a twin-engined cabin-fuselage biplane, the general lines of which, though more or less conventional, are clean-cut and pleasing.
   The pilot is provided with a roomy cockpit situated well back along the fuselage, where he has an excellent range of vision in all directions. Provision is also made in the same cockpit for a mechanic, who sits beside the pilot - there being plenty of room for both, their comfort having been well looked after. The various instruments and controls are well arranged within the cockpit.
   Extending from the pilot's cockpit to the nose of the fuselage, the passengers' cabin is exceptionally roomy and comfortable, providing accommodation for about ten passengers and measuring 20 ft. by 4 ft. by 6 ft. Windows round the sides and in the nose afford splendid vision, while there is plenty of head room, enabling the passengers to move about with ease. The ventilation of the cabin is good, and access is by a door on each side of the fuselage. These doors are in two parts, an upper sliding portion, which can be opened during flight, and a lower portion, which hinges down and forms a stairway into the cabin from the ground. A door at the rear of the cabin communicates with the pilot's cockpit, while two doors in the central part of the cabin enable the mechanic to get at the engines, via the wings, during flight. It should be mentioned that the cabin interior is free from all leads, petrol fumes, etc.
   The power plant - two 400 h.p. "Liberty" engines mounted on the lower wings - is well installed, each engine with the oil, etc., installation forming a separate unit, which can be easily and quickly removed or replaced. The fuel system, consisting of gear pump and gravity tank, is simple and reliable, and the risk of fire has been reduced to a minimum. The engines are carried by strong, rigid steel (channel-section) frames mounted on and projecting forward of the lower plane, and forming neat nacelles. The radiators are located at the rear of these nacelles, and can slide in and out for varying the amount of surface exposed to the air.
   As regards the structural features of the S.29A, the general construction, which is mainly of metal, is what might be termed "rugged" and simple, and is reported to be particularly promising from the production point of view. The main wing spars are built up of cold-rolled steel angles and channels forming I beams. A number of strong ribs, spaced about 3 ft. apart, and made of duralumin channels and ties 0.035 to 0.055 in. thick, connect the two main spars, with six false spars running parallel to the latter. Light intermediate ribs of 0.02-in. gauge channel section are riveted to the main and false spars.
   The tail plane and control surfaces - which are of ample proportions and easily operated - are also built up of steel and duralumin. The tail plane is adjustable as to incidence from the pilot's cockpit, while the rudders possess a somewhat novel feature. The two outer rudders (there are three in all) are single cambered, with the camber on the inner side - an arrangement that possesses certain advantages when flying on one engine. When both engines are on, the rudders in normal position neutralise one another, but if one engine is throttled down or goes out of action, the rudder in the slipstream of the engine still running will exercise a turning tendency opposing the turning effect of the engine.
   The fuselage is also built up of steel and duralumin. The longerons are of steel angles, and at the cabin portion the struts are steel channels, and of duralumin channels at the rear. Transverse bracing at the cabin is by means of gusset plates.
   A well-sprung, although somewhat narrow-track, landing gear is fitted, the shock-absorbers, giving a 12-in travel, being enclosed in the wing.
   The following are the general characteristics of the Sikorsky S 29A, together with particulars of two performance tests. The latter, it is interesting to note, were carried out by M. Sikorsky himself :-
   Span (upper) 69 ft.
   ,, (lower) 63 ft.
   Chord (upper) 10 ft. 3 ins.
   ,, (lower) 5 ft.
   O.A. length 49 ft. 10 ins.
   O.A. height 13 ft 6 ins.
   Total wing area 992 sq. ft.
   Area of tail plane 58 sq. ft.
   Area of ailerons 72 sq. ft.
   Area of elevators 50 sq. ft.
   Area of rudders 38 sq. ft.
   Wing section Sikorsky 18 (modified Gottingen 436)
   Incidence to thrust line 4 deg.
   Weight, empty 7,775 lb.
   Specified useful load 4,225 lb.
   Specified gross weight 12,000 lb.
   Wing loading 12-1 lb./sq.ft.
   Power loading 15 lb./h.p.
   Safety factor 4-5
   Airscrews Hamilton, 10 ft. 4 ins. dia., 5,ft. 10 ins. pitch

   Performance Tests. - Flights with one engine throttled. Right and left-hand turns were made at a barograph altitude of about 1,100 ft., with right engine all out and left engine throttled down to 750 r.p.m. Both right and left-hand turns were made with ease, but no steep banks were attempted. Data :-
   Weight empty (with water) 7,775 lbs.
   Disposable load of -
   9 men 1,530 lbs.
   108 galls, of petrol 648 lbs.
   9 galls, of oil 67 lbs.
   Equipment, etc. 241 lbs.
   Total disposable load 2,486 lbs.
   Gross weight 10,261 lbs.
   Initial recording barograph reading 900 ft.
   Final do 1,350 ft.
   Duration of climb, one engine. 15 mins.
   R.p.m. (right engine) 1,540.
   R.p.m. (left engine) 750.
   Average air speed during climb 70 m.p.h.
   Corrected climb, standard air at 1,125 ft. 28.4 ft./min.
   Temperature 38° F.

   Maximum Speed, Climb and Ceiling Test -
   12 men 2,040 lbs.
   128 galls, petrol 768 lbs.
   12 galls, oil 89 lbs.
   Total disposable load 3,082 lbs.
   Gross weight 10,857 lbs.
   Max. speed, corrected to standard air at 550 ft 111.2 m.p.h.
   Min. speed 55.6 m.p.h.
   Service ceiling 12,300 ft.
   Climb to 5,000 ft 8.8 mins.
   ,, 10,000 ft. 23 mins.
   R.p.m. up to 10,000 ft. 1,550 (R,); 1,525 (L.)
   ,, above 10,000 ft 1,525 (R.); 1,475 (L.)
   Observers during the performance tests reported that the control of the machine was exceptionally easy, and that excellent steadiness in flight was observed under severe weather conditions. Landings were always made with ease and comfort, and the get-away was rapid.
   Lieut. C. E. Archer, of McCook Field, reports on the Sikorsky as follows :-
   It is easy on the controls, a very slight movement of the wheel being required for straight flight or turns. Rigidity of the structure in flight particularly noticeable, there being not the slightest vibration in any part, although during the flight the engines were quite rough. Machine flew and actually climbed about 150 ft. in a very few minutes on one engine, and the take-off was made in 9 1/2 secs.
   In conclusion, M. Sikorsky states that the machine was built under somewhat difficult conditions, with very little shop equipment available. The entire design is, therefore, capable of refinement and could be lightened (without sacrificing strength) by some 750 lbs.